Field of the Invention
The present invention relates to an interchangeable lens apparatus and an image capturing apparatus which are used for a lens-interchangeable image capturing system.
Description of the Related Art
In interchangeable lens image capturing systems, in order to ensure compatibility of an interchangeable lens apparatus (hereinafter referred to as “an interchangeable lens”) and an image capturing apparatus (hereinafter referred to as “a camera body”), the interchangeable lens stores information on its image capturing optical system and sends the information to the camera body. The information sent from the interchangeable lens to the camera body contains focus sensitivity required to calculate a drive amount of a focus lens included in the image capturing optical system, depending on a defocus amount of the image capturing optical system detected in the camera body by a focus detection function such as a phase difference detection method. The focus sensitivity indicates a relation between a unit movement amount of the focus lens and a displacement amount of an image position (e.g., a ratio between the unit movement amount of the focus lens and the displacement amount of the image position); the image position is a position of an optical image formed by the mage capturing optical system. Dividing the detected defocus amount by the focus sensitivity as the ratio enables acquiring the drive amount of the focus lens required to acquire an in-focus state (see Japanese Patent Laid-Open No. 59-151116).
On the other hand, the camera body is classified into two types; one is a conventional-type camera body that performs focus detection by the phase difference detection method by using a signal acquired from a focus detection sensor independent of an image sensor for acquiring captured images; the other is a new-type camera body that performs the focus detection by using a signal acquired from the image sensor. The conventional-type camera body has a configuration that introduces, to the focus detection sensor, light (optical image) passing through the image capturing optical system and then being reflected by a quick return mirror. For this reason, many conventional-type camera bodies use a small focus detection sensor and thus these camera bodies can detect the defocus amount only in a limited image height range near a central area of an image capturing frame. In contrast, as disclosed in Japanese Patent Laid-Open No. 2010-025997, the new-type camera body is capable of performing the focus detection by using signals acquired from pixels arranged on an overall area of the image sensor. This capability enables detecting the defocus amount not only at low image heights near the central area of the image capturing frame but also at high image heights in a marginal area of the image capturing frame.
However, the inventor has discovered in the focus detection for an identical scene performed by the new-type camera body that there is a significant difference between the defocus amounts at the low image heights and those at the high image heights (which will be described later in detail). When there is such a difference between the defocus amounts at mutually different image heights, driving the focus lens by a drive amount calculated by using the focus sensitivity provided for the low image height with respect to the defocus amount detected at the high image height cannot acquire a good in-focus state at the high image heights.
On the other hand, Japanese Patent Laid-Open No. 59-151116 discloses an autofocus apparatus that reduces a period of time of focusing by setting the focus sensitivity depending on a focal length of a lens.
However, the autofocus apparatus disclosed in Japanese Patent Laid-Open No. 59-151116 is designed without consideration of a variation in the focus sensitivity depending on a position of a focus detection position at which an in-focus state should be obtained. The focus sensitivity assumed for this apparatus is a value corresponding to a center of an image surface, that is, an image height of zero. Applying the focus sensitivity depending on the image height of zero when the focus detection position is located at a position where the image height is not zero may decrease an in-focus accuracy.